Conventional engine valves (intake or exhaust) used to control the flow into and out of the cylinders of internal combustion engines, are controlled by camshafts that fix the amount of lift as well as the opening and closing times of the valves relative to a crankshaft position. While this may be generally adequate, it is not optimal, since the ideal intake and exhaust valve timing and lift vary under varying operating conditions of the engine. Variable valve timing and lift can account for such conditions as throttling effect at idle, EGR overlap, etc., to substantially improve overall engine performance. Although some attempts have been made to allow for variable timing based upon adjustments in the camshaft rotation, this is still limited by the individual cam lobes themselves.
Consequently, some others have attempted to do away with camshafts altogether by individually actuating the engine valves by some type of electromechanical or electrohydraulic means. These systems have not generally proven successful, however, due to substantial costs, increased noise, reduced reliability, slow response time, or increased energy consumption of the systems themselves. Further, although some systems allow for extensive control of valve timing, they are limited as with the conventional camshaft systems to a single valve lift distance thus not fully taking advantage of engine efficiencies that can be had, or variable lift is achieved with degradation in valve performance.
One type of electromechanical system attempted employs simple solenoid actuators. But these have proven inadequate because they do not create enough magnetic force for speed needed to operate the valves without an inordinate amount of energy input. This is particularly true in light of the fact that the force profile is not desirable. The magnetic force increases as an armature disk approaches the electromagnet, causing a slap at end of stroke, creating noise and wear concerns, but not much force is available for acceleration at the beginning of the stroke, creating slow response time. Further, they are typically limited to a single amount of valve lift.
U.S. Pat. No. 5,222,714 attempts to overcome some of the deficiencies of an electromagnetic system by providing a spring to create an oscillating system about a neutral point wherein the spring is the main driving force during operation, and electromagnets provide holding forces in the opened and closed position, while also making up for frictional losses of the system. However, this system is still not able to fully utilize the possible efficiencies of the engine. A major drawback is that although this system allows for extensive control of valve timing, it is limited as with the conventional camshaft systems to a single valve lift distance, thus not fully taking advantage of engine efficiencies that can be had.
Furthermore, the system may still suffer from some undesirable effects not present in prior cam driven systems. For instance, since the electromagnets act on the plate, not the valve head, thermal expansion of the valve stem and manufacturing tolerances can mean that when the plate is in contact with the magnet, the valve may not be fully closed. One way to avoid this problem is for the plate to be designed so that even under the worst condition a gap remains between the magnet and plate, with a large gap at the other extreme of tolerances. To account for this possible large gap then, the current must be increased to hold the plate against the spring with the large gap, increasing energy consumption and heat of the system, and making the actual seating force unknown for any given assembly. Further, to assure closing of the engine valve head with these tolerances, the engine valve can seat with substantial velocity, resulting in unwanted noise and wear.
A consistent, known seating force is desirable for closing the engine valve in its valve seat. Further, it is also desirable for the system to take into account manufacturing tolerances and temperature variations without having to significantly increase the power consumption of the actuator.
Hence, a simple, reliable, fast yet energy efficient actuator for engine valves is desired, with the flexibility to vary both valve timing and lift to substantially improve engine performance, without degrading valve performance with varying lift.